I just switched to calibre 0.7.45 and I just tried converting a couple books from epub to mobi and when the conversion is complete the calibre viewer is showing all the html tags in the mobi file. (ie. >div>, >h2>)

Everything looks fine in the epub file but the mobi is all messed up. Files that were converted in previous versions are fine but if I do the conversion from the epub again, I get all the html tags.

Is there an option in Calibre that I hit that made this happen or is this just a bug.

Create a new ticket in Calibre's bug tracker. Attach the files there. It's on different servers and a different site, different rules apply there.
Remember to include as much info about what goes wrong as you can, and/or link back to this thread.